The present invention relates to a process for the preparation of liposomal medicaments which are associates of at least one amphiphilic bilayer-forming substance and a solubilizing agent. These are formed in an aqueous phase optionally with the addition of pharmaceutical substances and/or of pharmaceutical auxiliaries.
Amphiphilic bilayer-forming substances are substances which have both polar (hydrophilic) and non-polar (lipophilic) properties and which form bilayers (double layers) in an aqueous phase.
Liposomes are spherical structures with a diameter of from 20 nm to a few .mu.m. They consist of at least one double layer and enclose a certain, aqueous volume. Depending on the number of double layers enclosing the aqueous inner volume, unilamellar (one double layer), oligolamellar (a few double layers) and multilamellar (many double layers) liposomes are defined.
In contrast to unilamellar liposomes, in which an aqueous inner volume corresponding to their size is enclosed, oligolamellar and multilamellar liposomes enclose several separate aqueous inner volumes according to the number of double layers.
On the basis of their physico-chemical properties and their structure, liposomes can be used as carriers for pharmaceutical substances. These substances are incorporated in or bonded to either the aqueous inner volume or the lipophilic double layers as a result of their hydrophilic and/or lipophilic properties.
When pharmacodynamically and/or biologically active bilayer-forming agents are used, the liposome itself is the medicament. The liposomal medicaments can be converted into appropriate galenical forms of administration, depending on the mode of administration.
For controlled medication, liposomal medicaments can be used for therapeutic and/or diagnostic purposes and as depot medicament forms. Furthermore, conversion of a pharmaceutical substance into a liposomal medicament can produce an improvement in the stability of the pharmaceutical substance and in the resulting galenical form of administration.
Known processes for the preparation of liposomes and liposomal medicaments include:
1. Shaking and/or acoustic irradiation of amphiphilic bilayer-forming substances, optionally with the addition of pharmaceutical substances and/or pharmaceutical auxiliaries, in an aqueous phase.
2. Injection of amphiphilic bilayer-forming substances, dissolved in organic solvents, such as ethanol or ether, into an aqueous medium, pharmaceutical substances and/or pharmaceutical auxiliaries optionally being present.
3. Acoustic irradiation of a system consisting of an aqueous phase and an organic phase, which contains amphiphilic, bilayer-forming substances, and removal of the organic solvent by evaporation, optionally in the presence of pharmaceutical substances and/or pharmaceutical auxiliaries.
4. Dissolution of amphiphilic, bilayer-forming substances in an aqueous medium using solubilizing agents, mixed micelles or associates being formed, and subsequent removal of the solubilizing agent from the aqueous medium by means of gel chromatography or equilibrium dialysis.
However, all of these preparation processes have at least one in most cases several of the following disadvantages. Thus, each severe preparation process, such as, for example, ultrasonic irradiation, inevitably leads to partial degradation of the amphiphilic, bilayer-forming substances and of the pharmaceutical substances, such as, for example, proteins and peptides, to be enclosed in the liposome. If organic solvents are used, removal of these from the liposomal medicament formed is only incomplete. Likewise, it is not possible using these processes to prepare liposomal medicaments which are homogeneous with respect to the degree of dispersion in particle size of the liposomes contained therein--vesicle sizes of 20 nm to several thousand nm thereby occur--and/or with respect to the number of double layers enclosing the inner, aqueous phase of the liposomes. Exact metering of the pharmaceutical substances present in the liposomal medicament is thus scarcely possible.
It is frequently only possible to use very dilute dispersions, so that, if the liposomal medicament formed is to be present in a concentration required for medication, it becomes necessary subsequently to concentrate the medicament by means of expensive processes, usually ultrafiltration.
The known processes are also unsuitable for the preparation of liposomal medicaments on production scales. Further separation processes, such as ultracentrifugation and/or fractional filtration, must in most cases subsequently also be carried out to achieve the indispensable increase in homogeneity, whether with respect to the degree of dispersion or to the number of double layers enclosing the aqueous phase of a liposome.
The in vivo properties of liposomal medicaments are decisively influenced by the degree of dispersion of the liposomes contained therein and by the number of double layers which enclose the inner aqueous phase of the liposomes. Thus, high blood level values are achieved over a relatively long period with homogeneous, unilamellar liposomes having a diameter corresponding to the maximum pore size in the sinusoidal capillary area (about 100 nm). In contrast, the blood level values of smaller or larger unilamellar liposomes decrease significantly, and, in particular, the liver level values correspondingly increase. There is also the possibility of controlling the distribution of homogeneous unilamellar liposomes in certain organs, such as the spleen, kidney or lung, via their size. The in vivo properties of polydisperse unilamellar liposomal medicaments cannot be characterized, so that controlled medication is not possible. The in vivo properties of multilamellar, in most cases extremely polydisperse liposomes is characterized in that the liposomes become concentrated, above all, in the liver, spleen and lung, which means that extremely low blood levels result.